We recently completed a study of the cytokinetics of repair, alterations in cell cycle parameters, and differentiation rate changes during skin regeneration and recovery after 3000 rep beta-irradiation to guinea pig skin. The study is being extended to include lower, more clinically relevant doses (1000 to 2000 rep beta) and determine the importance of survivors in the irradiated area as a possible source of cells for epithelization, as compared with cells migrating in from the radiation margin. Do the cell cycle parameters of such survivors differ from that of the latter? Do surviving hair follicle cells now play a role in epithelization? Does the cytokinetics of repair differ with decreasing irradiated area after 3000 rep beta, and is there a minimal area where radiation-induced hyperplasia becomes transient? Finally, how do cells in the healed hyperplastic epidermis (50 days or longer postirradiation) react to a subsequent irradiation and what is their cytokinetics of repair? This new information will be of value not only to those concerned with delivering fractional skin-sparing doses during radiation therapy, but to investigators who are interested in the general problem of wound healing. Since the majority of animals are shaved prior to treatment of the skin, studies have been included on the effect of this mild trauma on diurnal variations in labeling index (LI) and mitotic index (MI) of basal layer cells. Having used the intradermal route for injection of tritiated thymidine, a comparison is underway of this technique as compared to the interperitoneal route in obtaining values for LI and percent labeled mitoses in epidermis.